It is desirable, in designing a high power inverter, to avoid using turn-on and turn-off snubbers, thereby reducing cost and increasing reliability. This, of course, assumes that steps are taken to protect the switching transistors against over-voltage. Snubbers provide a path for inductive currents during transistor turn-off to avoid stressing the transistors with over-voltage due to the energy stored in the loop inductance during transistor forward conduction.
In order to avoid using turn-off snubbers, the circuit inductance has to be minimized. However, an inverter circuit requires more than minimal inductance to avoid switching problems at device turn-on. Specifically, a dc-to-ac inverter that is pulse width modulated so that transistors in the same inverter leg are alternately turned on, or a chopper where transistors in the same leg are alternately turned on, when supplying an inductive load, experience the problem that modern state-of-the-art switching transistors turn on so rapidly that it is expensive, if not impossible, to procure a coasting diode for the transistors (Coasting diodes are diodes connected in inverse parallel across the transistors and provide a current path in a direction opposite to that of the parallel connected transistor), which will accommodate the required forward current level and switch off sufficiently fast to prevent an excessive current overshoot without the use of expensive snubber circuits.
When, for example, the lower transistor in a leg of an inverter is off and the upper leg is to be turned on, the lower coasting diode carries full load current (assuming an inductive load). As the upper transistor turns on, the current in the lower diode decreases. While the stored charge is being cleared from the lower diode, the diode current overshoots and becomes negative and a low impedance path is created comprising the upper transistor and the lower coasting diode connected in series across the dc rails.
Fast, high voltage coasting diodes used for this duty often exhibit potentially disastrous "snap off" characteristics. If a coasting diode "snaps off" (goes rapidly from a reverse current due to stored charge to zero current) the large rate of change of the current can create a large voltage across the transistor in parallel with the diode and destroy the transistor.
Typical methods presently used for avoiding excessive turn-on collector current in the transistor switching devices during the coasting diode clearing interval employ inductive snubbers in the collector circuit and/or a careful matching of the dynamic switching characteristics of the transistor and its coasting diode.
It is an object of the present invention to provide a circuit for limiting peak turn-on stress imposed upon power switches in an inverter during coasting diode clearing intervals.
It is a further object of the present invention to provide a high power transistor inverter that does not require turn-on or turn-off snubbers.
It is a still further object of the present invention to provide a high power transistor inverter that develops power for the base drive circuit from the dc link.
It is another object of the present invention to provide one local power supply for the control logic and base drives of all the switching transistors in an inverter.
It is yet another object of the present invention to provide a power transistor base drive circuit that does not require isolating base drive transformers.